Floating membrane covers are mounted over settling ponds and clarifiers to contain and collect fermentation gases of mill effluents for example, and in many cases, to prevent the accumulation of rainwater inside these reservoirs. The maintenance of a membrane cover floating on a large wastewater reservoir represents certain difficulties in that the cover is exposed to the elements, and especially to wind uplift and wind-induced fluttering.
The fermentation of wastewater in a covered reservoir generates bio-gases that tend to create gas pockets under the cover. Following a rainfall, rainwater accumulates around the gas bubbles, thereby creating puddles, mounds and relaxed segments at random locations on the cover. These puddles and mounds catch the wind, creating uplifting forces on the cover. The relaxed segments also catch the wind and can even flutter in the wind. Such uplifting forces and fluttering movements promote the formation of waves along the cover and into the liquid under the cover.
The wind-induced forces and the movement of liquid under the cover causes tangential stresses and constant movement of the membrane itself. These movements and stresses can cause fatigue, localized elongation and can eventually rupture the membrane.
These inconveniences with floating membrane covers have been addressed in the past in different ways by different inventors. The following examples of floating membrane covers for covering large liquid reservoirs provide a good inventory of the prior art solutions to deal with wind forces on a membrane cover. In the following examples, weight lines, floating ridges and drains were used to reduce the formation of water puddles over the cover and gas pockets under the cover.
Examples of membrane covers mounted over industrial, municipal or agricultural reservoirs are disclosed in the following documents:    U.S. Pat. No. 3,980,199 issued to W. B. Kays on Sep. 14, 1976;    U.S. Pat. No. 3,991,900 issued to N. R. Burke et al. on Nov. 16, 1976;    U.S. Pat. No. 4,446,983 issued to D. H. Gerber on May 8, 1984;    U.S. Pat. No. 4,476,992 issued to D. H. Gerber on Oct. 16, 1984;    U.S. Pat. No. 4,503,988 issued to D. H. Gerber on Mar. 12, 1985;    U.S. Pat. No. 4,603,790 issued to D. H. Gerber on Aug. 5, 1986;    U.S. Pat. No. 4,672,691 issued to C. J. DeGarie et al. on Jun. 16, 1987;    U.S. Pat. No. 5,505,848 issued to R. Landine et al. on Apr. 9, 1996;    U.S. Pat. No. 5,587,080 issued to R. Landine et al. on Dec. 24, 1996;    U.S. Pat. No. 6,338,169 issued to C. J. DeGarie on Jan. 15, 2002;    U.S. Pat. No. 6,357,964 issued to C. J. DeGarie on Mar. 19, 2002;    U.S. Pat. No. 6,497,533 issued to C. J. DeGarie et al. on Dec. 24, 2002.
In another perspective, the following document discloses a membrane cover that can be inflated over its entire surface, or inflated along peripheral segments only, to remove wrinkles therein.
Accordingly, U.S. Pat. No. 4,139,117 issued to H.S. Dial, on Feb. 13, 1979, discloses a membrane cover mounted over a liquid storage reservoir. The cover is inflated by blowing air there under to remove wrinkles therein and to facilitate the flow of rain and water from melting snow off the membrane toward an appropriate drain. Alternatively, separate envelopes along the perimeter of the cover can be inflated separately to provide lateral tensioning of the membrane to eliminate the formation of wrinkles in the membrane.
In yet another application, U.S. Pat. No. 6,361,249 issued to D. G. Hodgkinson et al. on Mar. 26, 2002, discloses a negative air pressure cover for reducing the odours from a lagoon, a manure storage basin, waste water pond and other reservoirs of the like. This installation comprises a perforated pipe laid under a resilient membrane cover, along the perimeter of the cover. A pumping device is connected to the pipe for extracting air from under the cover through the pipe. The membrane cover in thereby held down by negative air pressure.
Similarly, when bio-gases are generated inside a covered reservoir, it is a common practice to withdraw these gases for treatment, and when methane gas is included, for burning these gases. In a gas-withdrawal installation, the bio-gases are drawn by a vacuum pump connected to a conduit communicating with the perimeter of the reservoir.
In that regard, another relevant installation was found in the prior art. This example pertains to a membrane cover that is inflated above the liquid surface of the reservoir and that is used as a gas storage for supplying combustible gases to a gas burning installation. Accordingly, CA Patent Application 2,379,590 filed by C. J. DeGarie et al. on Mar. 28, 2002, discloses an inflatable membrane cover mounted over a wastewater reservoir. The cover is used as a gas storage for accumulating the bio-gases being generated inside the reservoir. An anemometer and a pump are used to control the inflation of the cover according to the wind speed above the cover. Under high wind conditions, the cover is deflated to reduce the probability of the cover being damaged by wind induced stresses.
Although much effort have been made in the past to eliminate puddles and mounds on a membrane cover, the problem has never been solved entirely. FIGS. 1, 2 and 3 of the accompanying drawings have been prepared to illustrate a common problem with gas pockets being formed under a membrane cover. It has been found that when a vacuum pumping system (not shown) is connected to a gas outlet pipe 20 and is used to withdraw bio-gases from under a floating membrane cover 22, water puddles 24 tend to form on the,cover along the walls of the reservoir, as represented by a rectangular configuration 26 in FIG. 1 of the attached drawings. The presence of weight lines and a drain does not seem to prevent the accumulation of rainwater puddles 24 along this rectangular configuration 26. Eventually, these water puddles expand and connect with each other along the rectangular configuration 26, thereby sinking the cover near the walls of the reservoir, and blocking the migration of bio-gases from a central region of the reservoir to the gas outlet pipe 20. It has been found that an increase in negative pressure in the gas outlet pipe 20 causes the regions of adherence to expand.
Consequently, it is still a common practice for maintenance workers to use brooms and squeegees to push this water toward the drains. Such walking activities on a membrane cover wears the membrane material and reduces the life of the cover.
In a wastewater reservoir 30, as shown in a simplified manner in FIG. 1, the reservoir is covered by a flexible floating membrane 22. The bio-gases generated from the fermentation of the wastewater inside the reservoir migrates under the cover to the perimeter of the reservoir, as can be understood from FIG. 2, where it is drawn out by a vacuum pump (not shown) communicating with the gas outlet pipe 20. Weight lines 32 are normally laid on the membrane cover to keep the membrane taut and to promote the migration of bio-gases toward the perimeter of the cover. One or more drains 34 are also installed in the cover to evacuate rainwater.
It has been observed that when water accumulates in a random manner on a membrane cover, the combination of the vacuum under the cover along the perimeter of the reservoir and the water puddles cause the membrane to adhere to the surface of the liquid inside the reservoir near the sides of the reservoir. The vacuum under the cover, the water puddles and atmospheric pressure on the cover causes patches and strips of the membrane to adhere, such as a flap valve, to the liquid surface near the walls of the reservoir. These adhered patches and strips retain the membrane to the liquid surface of the reservoir and form basins along the walls of the reservoir to further retain rainwater along the walls of the reservoir. As more water accumulate on the cover, or when more vacuum is applied under the cover, more patches adhere to the liquid surface, thereby promoting the formation of gas pockets in a central area of the reservoir. The gas pockets in a central area of the reservoir push rainwater toward the sides of the reservoir, causing more or longer patches and strips of membrane to adhere to the liquid surface of the reservoir.
The root cause of this phenomenon is referred to herein as a flap-valve attachment phenomenon or a flap-valve attachment condition.
Referring now to FIG. 3, a flap-valve attachment phenomenon will be described as it is best understood. Generally, the vacuum pump is set to create a negative pressure in the bio-gas outlet pipe 20 and in the gas passage 36 around the perimeter of the reservoir 30. It has been found that this negative pressure causes the liquid level 38 in the gas passage 36 to rise slightly above the liquid level 40 at the center of the reservoir. This difference in elevation is shown in an exaggerated manner as ΔH in FIG. 3. This difference in elevation ΔH causes a region of adherence of the membrane over a distance ‘A’, which corresponds to the cotangent of the angle of contact of the membrane 22 against the surface of the wastewater, multiplied by ΔH.
This region of adherence ‘A’ is fixed relative to the wall of the reservoir 30, and its position and width depend on the tension in the membrane, the level of wastewater in the reservoir and the negative pressure in the gas passage 36.
This region of adherence ‘A’, and more particularly the atmospheric pressure 42 in that region ‘A’ causes a flap-valve attachment condition, which blocks the migration of bio-gases toward the gas passage 36 along the perimeter of the reservoir. Any increase in negative pressure P1 in the gas passage 36 worsens this flap-valve attachment condition. Any increase in the positive bio-gas pressure P2 in a gas pocket also worsens this condition.
As the bio-gases accumulate at the center of the reservoir, the rainwater 24 on the surface of the cover tends to accumulate along the rectangular region of adherence 26 as illustrated in FIG. 1. This accumulation of rainwater 24 away from the drain 34, increases the tension in the membrane 22 along the edges of the reservoir 30, and reduces the life of the membrane cover.
As it is best understood, these flap-valve attachment conditions are also initiated by the surface tension in the liquid inside the reservoir, and by a capillary effect between the liquid surface and the membrane material.
Accordingly, there remains a need in the industry for a better solution to reduce the formation of gas pockets under a membrane cover.